This invention is in the field of composite nanofiltration membranes, in particular composite nanofiltration membranes employing at least one lyotropic liquid crystal polymer porous membrane on a porous support.
Polymer membranes based on lyotropic liquid crystal (LLC) mesogens are of interest because of the ability of LLC mesogens to self-assemble into ordered, nanoporous aggregate structures in the presence of a solvent such as water. The aggregates can be relatively highly ordered yet fluid condensed assemblies with specific nanometer-scale geometries, known collectively as LLC phases (Gin et. al., “Polymerized Lyotropic Liquid Crystal Assemblies for Materials Applications,” 2001, Acc. Chem. Rec. 24, 973-980). LLC mesogens are amphiphilic molecules containing one or more hydrophobic organic tails and a hydrophilic headgroup. Surfactants can be classified as amphiphiles (D. Considine, ed., Van Nostrand's Scientific Encyclopedia, Seventh Edition, 1989, Van Nostrand Reinhold, New York, p. 861).
Polymer membranes based on LLC mesogens have been reported. Beginn et al. reported membranes containing ion-selective matrix-fixed supramolecular channels (Beginn, U.; Zipp, G.; Möller, M. “Functional Membranes Containing Ion-Selective Matrix Fixed Supramolecular Channels,” Adv. Mater. 2000, 12, 510). Solutions of 2-hydroxymethyl-[1,4,7,10,13-pentaoxacyclopentadecane]-3,4,5-tris[4-(11-methacryloylundecyl-1-oxy)benzyloxy]benzoate, a tris-methacrylated crown ether amphiphile, in a mixture of monomers, cross-linkers, and a photo-initiator were reportedly cast to thin films on a supporting porous filter (Pall Filtron NOVA membrane with maximum pore size of 10 microns). The mixture was subsequently cooled to −50° C. on a temperature-controlled aluminum block and then polymerized. The cross-section of the supported membrane reportedly showed that the support was completely filled with the cross-linked methacrylate. The supramolecular channels were reportedly formed by self-assembly of the tris-methacrylated crown ether amphiphile into long cylindrical aggregates with the crown ether moieties stacked parallel to the column axis and the polymerizable groups forming the shell of the cylinder.
Beginn et al. also reported membranes containing oriented supramolecular transport channels (Beginn, U.; Zipp, G.; Mourran, A., Walther, P., and Möller, M. “Membranes Containing Oriented Supramolecular Transport Channels,” Adv. Mater. 2000, 12, 513-516.). The membranes were synthesized by filing the 400 nm wide pores of a track-etched polyester membrane with a hot isotropic methacrylate solution of 2-hydroxymethyl-[1,4,7,10,13-pentaoxacyclopentadecane]-3,4,5-tris[4-(11-methacryloylundecyl-1-oxy)benzyloxy]benzoate, a tris-methacrylated crown ether amphiphile (60 wt.-%). The filled polyester membrane was cooled below the isotropization temperature of the lyotropic solution and the solution polymerized.
WO 98/30318 to Gin et al. states that polymer membranes can be formed from amphiphilic LLC monomers that will self-organize into stable, inverse hexagonal phases in the presence of pure water or other hydrophilic solutions. It was further stated that in situ photopolymerization of the hydrophobic tails into a heavily cross-linked network with retention of the template microstructure yields a robust polymer network with highly uniform pores arranged in a regular hexagonal array. Formation of a polymer film between two glass slides by photopolymerization of a LLC monomer mixture was reported. It was further reported that the film could be peeled off the glass slides in one piece.
U.S. Pat. No. 5,238,613 to Anderson reports polymeric membrane materials having a pore size between two nanometers and sixty microns. The porosity of the membrane materials is reported to be greater than fifty percent. U.S. Pat. No. 5,238,613 states that binary water/polymerizable surfactant bicontinuous cubic LLC phases could provide a route for membrane formation.
A need continues to exist for polymer membrane manufacturing technologies which allows control of critical structural features such as pore size, pore architecture, and pore density in the nanometer size regime. A need also exists for polymer membranes for which these critical structural features can be controlled on this extremely important size scale.